That dietary fats alter susceptibility to chemically induced cancer has been demonstrated through epidemiological studies of human populations and experimental studies of laboratory animals. The mechanism by which responses to chemical carcinogens are altered is unknown. However, reasonable assumptions are that diet induces changes which increase or decrease tissue reactivity to carcinogen, or that diet influences activity of the mixed function oxidases which are responsible for the metabolic activation or detoxification of carcinogen. The objectives of this research are to determine the influences of type of dietary fat on the activity of the mixed function oxidases of rat liver as these oxidases participate in the metabolic activation and/or detoxification of the potential carcinogens 3,4 benzo(a)pyrene, 2-acetylaminofluorene and dimethylnitrosamine. Using hepatic microsomes from control and carcinogen pretreated rats fed the specified saturated or unsaturated fats for one to six weeks, the in vitro metabolism of each carcinogen will be assessed. The apparent kinetics for the formation of each metabolite will be individually calculated. Correlation of metabolite and kinetic data with alterations in mutagenicity to appropriate strains of Salmonella using the reverse mutation system of Ames (1) will indicate the significance of changes induced by dietary fats in chemical carcinogenesis. Correlation of metabolite and kinetic data with concentration and activity of components of the mixed function oxidase system will identify underlying physiologic processes altered by dietary fats.